1. Field of the Invention
This invention relates generally to lead-acid batteries and more especially to sealed lead-acid batteries (SLAB), recombinant and valve regulated lead-acid batteries (VRLAB). More particularly, this invention relates to a system for maintaining intimate plate-to-separator contact during a battery's lifetime.
2. Related Art
Lead-acid battery cells comprise one or more positive plates alternating with one or more negative plates with a separator in between adjacent pairs of plates. The active ingredient on the positive plates is lead dioxide; the active ingredient on the negative plates is spongy lead. A number of materials have been employed as separators. At the present time, separators are typically made of a compressible pad or sponge of an absorptive glass fiber material. This absorptive glass mat (AGM) not only provides electrical separation between the positive and negative plates, it also absorbs and contains the acid electrolyte (H.sub.2 SO.sub.4 +H.sub.2 O) that the cells require for operation.
In order to get each of the adjacent positive and negative plates close together, the plates and separators are assembled with the plates standing and compressed between the walls of the battery. This arrangement compresses the separators. AGMs absorb more electrolyte when compressed. Each AGM has a specific compressive range in which it will absorb a maximum amount of electrolyte. Generally, the AGMs are compressed before the electrolyte is added. With the exception of flooded recombinant batteries, all of the electrolyte in a SLAB provided with AGMs is normally contained in the AGMs, and in the plate pores.
Compressing an AGM separator the optimum amount not only allows the absorption of more electrolyte, it also provides good contact between each separator and its adjacent plates. It is extremely important that each separator completely contact the entire surface area of each adjacent plate. This plate-to-separator contact provides an ionic conduction path between the plates and through the separator.
It has been found that any loss of contact between plate surfaces and separators results in an immediate degradation of cell performance and life. After such a loss of contact, the battery will have a lower discharge capacity, a loss in the number of amps out per square inch of battery plate surface area and increased internal resistance. Such a loss of contact all too often occurs while recharging a lead-acid battery, especially a sealed VRLAB. Lead-acid batteries produce gas during recharge. This is particularly true at a voltage above 2.35 volts per cell, a level that must be reached to fully recharge a lead-acid battery.
Since a VRLAB is sealed, the battery case traps the gas produced during recharge permitting no means of escape, except a safety valve. The increasing volume of gas increases the gas pressure within the battery case. The increased gas pressure, even below the pressure required for release by the safety valve, expands the battery case. Such expansion frequently causes a loss of contact between the plate surfaces and the separator surfaces. This loss of contact can cause an immediate degradation of cell performance and life.
Numerous attempts have been made to prevent case expansion. These attempts have included providing ribs on the battery case or thickening the walls of the case. These attempts, however, have not completely solved the problem; moreover, they add undesirable weight and cost to the battery.
Expansion of the battery case is not the only problem gassing causes. Bubbles rising in between standing plates contact the active material on the plates. This contact strains and loosens the active material near the surface of the plate. This strain may flake the active material and cause it to fall between the plates or rise with the gas stream. This can cause short circuits between the plates. Vibration and shock loads also contribute to the shedding of active material.
Electrolyte will settle within each individual standing AGM. Sulfuric acid (H.sub.2 SO.sub.4) has a greater specific gravity than water (H.sub.2 O) and will settle, over time, to the bottom of a standing AGM. Such settling will cause the lower portion of a standing AGM to have a higher specific gravity than the upper portion of the same AGM. As the acid concentration in the upper portion of an AGM becomes too low, cell resistance increases in that area, allowing preferential discharge to occur in the more conductive areas of the cell. This results in uneven and overdischarge of the paste in the more conductive area. The cell capacity as a whole decreases when electrolyte settles in the individual AGMs.